Even though the principles of the present invention apply to various types of electronic assemblies, the novel features of the present invention, both as to organization, content and function, are best understood and appreciated by reference to an electronic controller for variable speed operation of polyphase AC motors. Before describing the controller assembly of the present invention in detail, it will be helpful to understand its function, circuit elements and current packaging techniques.
It is known that variable AC motor speeds are achieved by varying the frequency and voltage supply to the motor. Means, therefore, must be provided to continually generate frequencies and voltages consistent with the desired speed of the motor. FIG. 1 illustrates a typical controller block diagram consisting of solid-state electronic control circuits 10, 11 and 12 and power semiconductors 14, 15 and 16 generally consisting of diodes, switching transistors, thyristors, resistors, and capacitors as well as filter elements 18 consisting of large capacitors and inductors.
Typically in these circuits, all circuit elements are functionally interdependent, but because of their physical incompatibility, both as to size and heat dissipation, current packaging techniques employ separately constructed mechanical entities for the electronic control, the power semiconductors and the filter elements. Additionally, power semiconductors such as power switching transistors and rectifiers which generate significant amounts of heat must be conductively and/or convectively cooled to maintain junction temperatures within specified limits for proper operation. Presently this necessitates mounting the power semiconductors directly on bulky metallic heat sinks, in most cases precluding their physical integration with control circuit elements on printed circuit cards and thereby resulting in complex interconnection wiring networks. The increased potential for failure resulting from complex wiring runs and extensive use of connectors means significant reduction in reliability.